1. Field of the Invention
The present invention relates to dimming operations within an LED system. More particularly, certain embodiments of the present invention relate to controlling regulation within an LED system such that a dimming operation results in a relatively smooth transition of light intensity with minimal flickering.
2. Background of the Invention
The benefits and wide-range applicability of LEDs in today's lighting systems are now realized and recognized by those skilled in the art. For many years, halogen-based lamps were the primary light source implemented within lighting systems. Over the past years as LED technology has developed, the advantages of LEDs over halogen lamps have become increasingly apparent. When compared to halogen lamps, LEDs are relatively smaller, and have a longer operating life. Another important difference between halogen bulbs and LEDs is the significantly less amount of power required by LEDs to operate. For example, a halogen lamp may operate within a range of 20-50 Watts and an LED at about 5-15 Watts.
When LEDs are used for lighting applications, a cluster or an array of LEDs is used to achieve the requisite brightness and other desired lighting characteristics. One or more LED drivers are used to effectively control the electrical characteristics of the array of LEDs to suit the lighting. An LED driver is a self-contained power supply that has outputs matched to the electrical characteristics of the array of LEDs.
Many LED lamps are powered in the same way as other lighting applications, namely, starting with and using an alternating current (AC) power source. Depending on the geographic location or application, the AC source could range between 100V and 240V. The frequency of these AC sources ranges between 50 Hertz and 60 Hertz. To meet energy star requirements for LED lighting applications, the required power factor has to be greater than 0.7 in residential lighting applications and greater than 0.9 in commercial lighting applications.
In applications where the power levels are higher than 25 Watts, an active power factor correction circuit is typically used to provide a regulated high voltage DC bus. This regulated bus is used to power the LEDs by a power conversion circuit. This power conversion circuit may be an isolated topology or non-isolated topology.
FIG. 1 illustrates a prior art LED system in which dimming functionality is provided. As shown, an AC input 110 is fed into a standard incandescent or trailing edge dimmer 120 that can reduce the power supplied to the LED lamp by attenuating the power on the AC signal used to drive the LED lamp. In some applications where there is no need for dimming, the AC input 110 may feed directly into the LED lamp.
A diode bridge 130 converts the AC signal into a corresponding DC signal having a current that will be used to drive the LED lamp. The output of the diode bridge 130 is coupled to an EMI filter 140 that suppresses conducted interference on the DC power line feeding into the LED driver 150. The LED driver 150 effectively regulates the current being delivered to an LED array 160 to ensure a preferred performance of the LEDs within the array.
Maintaining proper power factor correction within a dimmable LED system presents a difficult problem. The dimmer 120 causes clipping on its output signal, which may result in the LED driver 150 to inappropriately regulate the current being drawn into the LED array 160. In particular, improper regulation may result in voltage overshoots or undershoots in the LED driver signal which may cause the LED lamp to flicker during a dimming operation. The power factor is not maintained while dimming and is only maintained without the dimmer. The power factor requirement is typically needed when there is no dimmer.
This issue is particularly problematic in situations where an LED lamp is installed within a retrofitted dimmable lighting system. Examples of such retrofitted dimmable LED systems include both standard incandescent dimmable lighting systems as well as trailing edge dimmers integrated within lighting systems. What is needed is a system and method that provides a smooth dimming operation in retrofitted LED lighting systems.